1. Field of the Invention
The present invention relates to a method for measuring the lifetime of a semiconductor material and an apparatus therefor, and in particular, to a method and a device for determining the lifetime of the semiconductor material in which pulse energy is injected into the semiconductor material in order to generate a particular attenuation of a carrier, and by measuring the attenuation characteristics of the carrier, highly reliable and effective detection of a defective semiconductor material crystal and a very small heavy metal taint is enabled.
2. Description of the Prior Art
Semiconductor materials, such as Si, Ga and As, are processed according to several hundreds of processing steps, including raw material preparation steps through to semiconductor device manufacturing steps. These raw material preparation and the semiconductor device manufacturing steps include material washing, impurity diffusion, thermal treatment, patterning and etching, and each disadvantageously introduces the possibility of the generation of defective crystals and heavy metal taints in the semiconductor material. Furthermore, since the number of such troublesome processing steps is very high, the control or management of these steps, which necessitate many laborious and precise operation, is very difficult.
Nevertheless, it is inevitable that some semiconductor materials or chips having defective crystals and/or heavy metal taints or pollutants will be produced at a certain rate. As a result, a number of devices for identifying defective crystals and fine metal taints in semiconductor materials or chips have been widely marketed. A general and conventional process for measuring and analyzing defective crystals in the semiconductor materials or chips has been used for determining the respective lifetimes of the chips.
FIG. 1 is a block diagram of the structure of a prior art apparatus for measuring the lifetime of a semiconductor material.
As shown in FIG. 1, microwave energy generated by a microwave oscillator 1 is guided by a magic tee 4 to a wave guide 8 via an impedance matching device 2 and an E-H tuner 3, and is irradiated onto a semiconductor material 10 which is the object of the measurement. The microwave energy a irradiated onto the semiconductor material 10 is reflected from the near surface of the material 10, from within the material 10, and from the reverse surface of the material 10, respectively, and returns to the magic tee as microwave energy b which is guided by the magic tee 4 to the E-H tuner 6 and detected by a detector 7.
The principle of the lifetime measurement will now be explained referring to FIG. 2, where the reference a denotes microwave energy which is constantly applied to the semiconductor material 10. Carriers are operatively produced when external energy is fed to the semiconductor material 10 in the form of pulses from a laser diode 9 at a time of the measurement. Since this area producing the carriers is equivalent to the semiconductor turning into a conductor, and since the microwave energy is reflected from the produced carriers (microwave energy is reflected 100% on a metal), the reflection of the microwave energy is detected as a temporary increase in the region. The chronological change of the increased microwave energy coincides with the chronological attenuation waveform of the produced carriers. Therefore, the crystal in the semiconductor material 10 can be evaluated by measuring the attenuation waveform (lifetime) of the produced carriers.
The respective views FIGS. 3A to 3C show examples of microwave irradiation set ups of a conventional apparatus for measuring the semiconductor lifetime. In particular, FIG. 3A depicts a microwave irradiation set up provided with a measurement table 21 made of non-metal material and a waveguide 8 for emitting and receiving the microwave energy, and a semiconductor material 10 to be measured being placed on the measurement table 21. FIG. 3B shows another microwave irradiation set up provided with the waveguide 8 described above and a measurement holder 22 made of non-metal material on which a semiconductor material 10 to be measured is held, the measurement holder 22 being used in place of the measurement table 21 shown in FIG. 3A. FIG. 3C shows still another example of a microwave irradiation set up having a metal plate 23 from which the microwave energy reflects, in addition to the measurement table 21 having the semiconductor material 10 placed thereon and the waveguide 8.
A contactless inspection method, such as a DLTS method (Deep Level Transient Spectroscopy), has been widely employed to detect fine or very small metal taints in a semiconductor material. According to the DLTS method, a diode is formed on a substrate and a voltage is impressed on the diode, generating a response to the impression after the voltage is cut off. The response as shown in FIG. 13D is measured or determined in the form of a changed amount I in the electric signal between the instants t.sub.1 and t.sub.2. The relation between the values of the changed amount I and temperature are plotted to determine the degree of metal taints using particular thermal peak values of the changed amount I based on various metals.
Although the object materials of the lifetime measurement are usually semiconductor materials, such as a Si-wafer, the resistivity of the materials ranges fairly extensively depending on the usage of devices.
The waveguide 8 which is used in the prior art lifetime measuring apparatus can be equivalently replaced with a distribution circuit as shown in FIG. 4A. If the distribution circuit is terminated with a terminal resistance Z.sub.1, a reflected signal corresponding to the terminal resistance Z.sub.1 is produced as shown in FIG. 4B. Accordingly, in the case of the apparatus shown in FIG. 1 for example, the horizontal axis Z in FIG. 4B may be replaced with the resistivity .rho..sub.s since the terminal resistance Z.sub.1 is equivalent to the resistivity of the Si-wafer. The signal of the reflected microwave energy measured in the regions a, c and b corresponding to the resistivities .rho..sub.a, .rho..sub.o1 and .rho..sub.b shown in FIG. 4B becomes as shown in FIGS. 5A, 5B and 5C, respectively. The region c is where the measurement is impossible. In other regions, such as d, e and f, the reflected microwave signals to be measured are greatly influenced by the non-linear characteristics of the above mentioned waveguide to thereby have significantly changed in signal intensity and deteriorated in data reproducibility. The prior art method is problematic since it cannot measure some of the object materials with a high reliability.
From another standpoint the prior art is problematic in that according to the conventional process for analyzing a character of the semiconductor material or for locating any defective crystals as shown in FIG. 3A, the effective amount of the reflected microwave signal is small due to the effects of the microwave energy a' passing disadvantageously through the semiconductor material 10 and the microwave energy b' reflecting from the measurement table 21. According to the conventional device as shown in FIG. 3B, the microwave energy b has a small S/N ratio value because the microwave energy passes through the semiconductor material 10 and is reflected on the measurement holder 22. In order to improve the S/N ratio of the signal by making the microwave a' passing through the semiconductor material 10 reflected on the metal plate 23 so as to increase the volume of the microwave b reflected from the semiconductor material 10, the device shown in FIG. 3C is utilized. This device essentially has disadvantages in that the signal output is unstable in its amplitude and period since the two microwave signals overlap in their phases. Especially, with regard to the device shown in FIG. 3C, the waveshape of the reflected microwave signal changes as shown in FIGS. 5A to 5C according to respective positions d.sub.1, d.sub.2 and d.sub.3 of the metal plate 23 as shown in FIG. 6, thus disadvantageously generating non-effective signals. In brief, the reflected microwave signal is apt to change according to the particular thickness of the semiconductor material 10 to be measured, the position of the metal plate 23, the thickness of the measurement table 21, the positional relationship between the measurement table 21 and the waveguide 8, and the like, thus generating data of little reliability.
It is noted that the amount of the reflected microwave signal is very small when a short lifetime of the semiconductor chip is measured, so that the reliability of data deteriorates because that the microwave signal is amplified through an amplifier and the lifetime data is electrically delayed.
From still another standpoint the prior art is problematic, in that according to the conventional inspection method a contact breakage inspection must be carried out for the semiconductor chips or material. While the conventional process for measuring the lifetime of the semiconductor material has considerable positive achievements concerning the measurements of defective crystal lattices, metal taint O.sub.2 swirls and the like, it has problems in the inspection of very small metal taints and surface and bulk lifetimes. Accordingly, the DLTS method of a breakage type has been used unavoidably to detect very small metal taints.
It is apparent from above that it has not been possible to detect very small metal pollutants in semiconductor chips using a non-contact method.